Jitter measurements for repetitive clock signals

ABSTRACT

A system and a method for measuring and quantitatively analyzing the jitter in repetitive electrical signals in a chip using a tester are described. The tester sorts chips based on the jitter measurements, thereby eliminating the need for external instrumentation. The waveform is sampled by the tester at various points of a period over a large number of periods and results are collected. The data is analyzed to determine the total range where the waveform is found to undergo a transition. The transition area is further analyzed to pinpoint the precise location of the transition for each period of the repetitive waveform. The data is used to quantify the jitter by means of statistical analyses, the results of which are used by the tester to sort the chips by comparing the calculated jitter characteristics to predetermined criteria.

BACKGROUND OF THE INVENTION

The invention relates to a system and method for measuring jitter inrepetitive electrical signals, and more particularly, to measuring andquantitatively analyzing the jitter in clock signals using a tester toeasily sort chips based on the jitter measurements, thereby eliminatingthe need for external instrumentation.

A phase-locked loop (PLL) is a component that is commonly used in manycircuit designs. A circuit of this kind typically synchronizes an inputsignal to an output signal. It operates such that the output signaltracks the input signal in phase or relative position of the waveform.The frequency can be the same, or by means of a divider or multipliercircuit, the input and output frequency may differ while still beingsynchronized in-phase. Jitter of the output of the clock signal in a PLLdetermines the quality and even the performance of the output signal andof any circuit depending thereof. Therefore, jitter measurements in aPLL become indispensable for any circuit design. Such PLL circuits haveproven to be excellent vehicles for determining the presence of jitterand lend themselves as vehicles for sorting good chips from bad ones.

It is well known that the semiconductor industry constantly strives toachieve higher processing speeds in connection with computers,telecommunication equipment, and the like. The common factor whichextends to all the technological fields strives to achieve an everincreasing transfer of large volumes of data at high speed. By way ofexample, microprocessors move data at speeds which are based on highfrequency clocks that determine the rate at which electrical signals areclocked through the circuitry.

FIG. 1 shows a basic block diagram of a PLL. An input signal and anoscillator (feedback) signal are fed to a phase detector. The input isthe signal to be synchronized with, while the feedback from the voltagecontrolled oscillator (VCO) is the output signal to be synchronized withthe input, provided the circuit operates correctly. The phase detectoroutputs a voltage through the loop filter that is proportional to thephase difference between the input and feedback waveforms. Thisproportional voltage is fed to the input of the VCO. The VCO frequencyis controlled by the input voltage and its starting frequencyapproximates the input frequency of the phase detector. If the input andoutput are out-of-phase, the voltage from the phase detector causes theVCO output to shift and reduce the phase difference between the inputand output. In this manner, the phase detector constantly monitors andadjusts the frequency of the VCO to maintain the phase differencebetween the input and output at zero. When the phase difference betweenthe two signals is zero, the output is said to be locked.

A PLL is commonly used to stabilize the clock signals, multiply theirfrequency, synchronize different parts of the circuit, synchronize onechip with another, clean up the signals, and numerous other similarapplications. It offers a clean and stable reference signal. Thebenefits of a PLL depend on many factors, such as how quickly itresponds to differences in its phase and adjust its output, what rangeof input frequencies it handles without losing its locking capabilities,how much noise or jitter exists on the output signal, and the like.

Jitter is a measure of the timing instability in a waveform or circuit.It is the variation from an ideal waveform that is caused by electricalchanges, design problems, characteristics of components in a system, orother similar problems. Unexpected changes in frequency, pulse length,and phase are indicative of the presence of jitter. Jitter becomes moresignificant at higher speeds since the same jitter comprises a largerproportion of the overall signal. Circuits that are time dependent,particularly when they operate at high frequencies, are especiallysusceptible to problems of jitter.

Microprocessors typically use an on-chip clock signal provided with aPLL. Because multiprocessor systems are commonly used, the timing ofevents is critical. Conventional systems are configured having aplurality of processors running in parallel, sharing and swapping dataand commands to accomplish the computing task. Data is transferred viadata busses which are synchronized between the many processors to ensureproper data transfer. Any problem with the data bus results in incorrectdata at the other end. Thus, PLL is advantageously used to synchronizedifferent chips attached to the data bus.

As mentioned previously, PLLs are particularly susceptible to jitter.Small amounts of jitter may go unnoticed, but at high speed, jitter onthe PLL causes data transfer errors. Redesigning the PLL or the systemmay normally be an option, but often the problem is not discovered untilthe system is shipped to a customer when it is no longer feasible toredesign. A way to measure and categorize the jitter is the next bestoption so that only good chips are used to ensure a good product.

Signal integrity analyzers and other special equipment exist to performa quick jitter analysis of signals, but this approach is both costly andcomplicated, especially in a manufacturing environment. Added to thecost of the equipment there is additional time and cost associated toconnecting other components to an existing tester, integrating systems,and ensuring proper operation. Preferably, the best approach is to usean existing production logic tester without any additions. It lowers thetest cost, reduces the number of necessary components, and allows theuse of equipment as it exists

OBJECTS AND SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide a method formeasuring jitter in repetitive signals, such as clock signals, usingonly a tester, and without requiring any added external instrumentation.

It is another object to provide a process for quantitatively analyzingthe jitter to sort chips as good or bad based on the results of thejitter measurements.

The invention provides a method for testing chips by measuring jitter onrepetitive waveforms generated by a circuit integral to the chip, thewaveforms being inputted into a tester coupled to the chip, the methodincluding the steps of: a) determining where the waveform generated bythe circuit undergoes a transition; b) taking a plurality ofmeasurements from the waveform at periodic time intervals, themeasurements being taken in a region where the waveform undergoes thetransition; c) analyzing the measurements taken from the region toquantify the transition and calculating a jitter on the waveform; and d)sorting the chips according to the jitter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and whichconstitute part of the specification, illustrate presently preferredembodiments of the invention which, together with the generaldescription given above and the detailed description of the preferredembodiments given below serve to explain the principles of theinvention.

FIG. 1 illustrates a conventional PLL circuit used for illustrativepurposes as a source of repetitive electrical signals.

FIG. 2 is a flow chart that illustrates the preferred embodiment of thepresent invention.

FIG. 3 is a waveform affected by jitter, wherein measurements are takenin three separate regions.

FIG. 4 illustrates the use of a tester to measure the jitter in signalsgenerated by a PLL circuit integral to a microprocessor.

FIG. 5 is an example of an ideal waveform outputted by the PLL of FIG.4.

FIG. 6 shows the relative positioning of the ideal waveform of FIG. 5showing how the pulses are displaced as a result of the jitter.

FIG. 7 illustrates two examples of the ideal waveform being measured,wherein the dotted lines indicate points at which the waveform is beingsampled.

FIGS. 8-11 illustrate a data summary of the jittering waveform used toconstruct Table 1 using the number of high measurements, the number ofsamples given and the probability of the measurements being at high inorder to calculate the jitter based on the edge locations of the idealversus the jittering waveform.

FIGS. 12A-12B show two waveforms, respectively sampled at 10 pointsillustrating transitions from a low to a high in the absence andpresence of jitter, i.e., in an ideal and a jittering waveform.

FIGS. 13-14 respectively show a waveform spanning over 1 million periods(FIG. 13), and the same waveform represented as 1 million individualperiods (FIG. 14).

FIG. 15 is a chart plotting the number of highs vs. measuring pointstaken at 10 picosecond intervals for 1 million periods sampled at eachposition, and a second plot superimposed thereon showing thedistribution of the jitter from multiple waveforms.

DETAILED DESCRIPTION

The invention provides a solution to the above problems by way of anovel method for measuring the jitter and assessing its effect. Theensuing analysis and evaluation will be described using for illustrativepurposes only, a Teradyne tester J973. The concept applies to other testplatforms besides the Teradyne tester and other waveforms besides PLLclock signals.

The basic flow chart representing the preferred embodiment of thepresent invention is illustrated in FIG. 2. Shown are the actual stepsto perform the necessary measurements. Alongside with the step embodyingthe present invention will also be shown accompanying examples toprovide a proper understanding of what happens at the various steps.

While the invention is preferably used to measure the jitter on anyrepetitive pulse string, for illustrative purposes, a phase locked loop(PLL) circuit will be used. The measurement of jitter will be performedon the tester, as previously stated.

The tester is used primarily to probe the chip and measure the output ofthe PLL integral to a microprocessor. The tester is configured so thatit takes measurements at any point within the period and handles anyvoltage at a high or low value. For simplicity, the tester is preferablyprogrammed such that for a 1.2 volt PLL waveform any voltage above 0.6volts (as the high value), and below 0.6 volts (as a low value) will bemeasured. The tester is also programmed to measure any point within aperiod, preferably with a 10 ps resolution. It is further necessary thatthe microprocessor be powered and fully operational and that an outputwaveform be generated by the PLL.

The flow chart will now be described in detail with reference to FIG. 2.

A measurement point is initially set (Step 101). It provides a referencethat pinpoints the location within the period at which the measurementpoints are to be taken. The tester is set to take measurements anywherewithin the period. In the first occurrence through the flow, this stepsets Measurement Point 1 (see FIG. 3).

In Step 102, the tester samples the waveform at the location specifiedin Step 101 by way of hardware that is configured to take themeasurement at a precise location within the period, and at a thresholdspecified to differentiate between a low and high value. This thresholdshould approximately be one-half the maximum voltage of the waveform andmust be constant throughout the duration of the jitter measurement.

Next, in Step 103, the measured value is checked. Therein, it isdetermined whether the value measured is above or below the thresholdspecified in Step 102. If it is above the threshold, it is labeled ahigh. If it is below, it is labeled a low. During this step, a decisionis made. If the waveform is found to be at high, a branch to step 104takes place. Otherwise, the flow moves to Step 105, omitting Step 104.

In Step 104, the total number of high measurements is incremented. Thusfar, only the measurement point specified in Step 101 is accounted for.No account is made of any number of highs found at any other measurementpoint.

In Step 105, a check is made to determine whether all the measurementsare accounted for. This number represents the total number of themeasurements that are to be taken at the point specified in Step 101.Alternatively, the number of measurements may represent the number ofperiods to be sampled. By way of example, as will be shown hereinafterwith reference to FIG. 14, a total of 1 million periods are sampled.Thus, in this step it is determined whether the 1 million samples havealready been taken.

At Step 105 a new decision is made. If the prescribed number ofmeasurements has not yet been completed, the chart returns to Step 102,taking additional measurements at the same measurement point onsubsequent periods. However, if the number of measurements coincideswith the number of periods required, a branch to Step 106 takes place.

In Step 106, the measurements are adjusted. Thereat, the number of highsthat were accumulated from previous measurements at the MeasurementPoint specified in Step 101 is determined and adjusted as will bediscussed hereinafter with reference to Table 2. The total number ofmeasurements corresponds to a number found in the “Probability of aHigh” column in Table 2. The adjustment provides an equivalent number inthe “Jitter” column. This number is derived by taking the difference ofthe current measurement and subtracting thereof the precedingmeasurement. For instance, if the total for the present measurementpoint is X and the preceding measurement point has a number of highs isY, then the adjusted measurement is X-Y.

In Step 107, the adjusted measurement (X-Y) from Step 106 is stored inan array. The actual measurement X from Step 106 is temporarily saved sothat it can be used to adjust the next number. It is this adjusted valueX-Y that is used to calculate the jitter.

In Step 108, a decision branch determines whether additional measurementpoints are necessary. For instance, and with reference to FIG. 3 it isshown that 250 measurement points are taken across each period.Accordingly, in Step 108 it is determined whether measurement point 250has been reached. If the answer is YES, the process comes to an END.Otherwise, a branch to Step 109 takes place and the process continueswith additional measurements taken. Upon reaching END, the data isstored in the array to be used for subsequent data analysis.

In Step 109, the process returns to the beginning of the process, takingmeasurements at a new location, i.e., by incrementing the measurementpoint within the period.

Still referring to FIG. 3, if the measurements were taken at MeasurementPoint 1, a new set of measurements is taken at Measurement Point 2. Thisstep simply determines the next location where measurements within theperiod are to be taken. Then, the process returns to step 101 where thisvalue is transferred to the tester (FIG. 4) and the waveform ismeasured.

As previously described, the jitter is defined as a deviation in thewaveform pulse length or the phase from an ideal waveform. For referencepurposes, an ideal waveform is shown with reference to FIG. 5. Theperiod will always have the same length, the same applying also to theedge placement.

The waveform shown is preferably used for comparison to the waveformsoutputted by the PLL. A reference point T₀ is labeled in each periodthereof. This point is preferably selected to coincide in each periodwith the transition from low to high. Hereinafter T₀, when referenced,is intended to describe the location of the transition point in an idealwaveform. For a non-ideal waveform, the transition point will occur atsome point other than T₀. The jitter of the non-ideal waveform isdetermined by the distance of the transition point from T₀ within thesame period.

It is necessary, therefore, to define an ideal waveform and T₀ so thatthe jitter of the measured waveform may be calculated by referencethereof. In FIG. 4, there is no ideal waveform referenced by the testerthat is associated with the waveform outputted from the chip. The testersamples a plurality of waveforms, and an ideal waveform is derivedthereof. For illustrative purposes, referring to the Gaussian curveshown in FIG. 15 labeled “Unique Edges (Jitter)” and which will bedescribed in more detail hereinafter, there is shown a graph of thedistribution of multiple waveforms. The center or mean value of thecurve is arbitrarily defined as T₀ or as the transition point of theideal waveform. Any other waveform measured will have its jittereddefined in relation to T₀. In this manner, it is not necessary to supplyan external reference signal to the tester since the tester itselfprovides the reference. The ideal waveform will be defined after themeasurements are collected.

Jitter causes a lateral displacement of the edge of the waveform. It iscaused by a change in the period or simply a phase shift. In eithercase, the edge placement of a jittering waveform differs from the edgeplacement of the ideal waveform, as seen with reference to FIG. 6.Therein is shown an ideal waveform at the top portion of the figure andtwo pulses below, i.e., pulses A and B. The pulses display differentkinds of jitter. In pulse A, the rising edge matches the ideal waveformbut the falling edge is delayed. In pulse B, both the rising and fallingedges are delayed. Either edge of the pulse moves in either directionwhen the waveform jitters. It is this movement that is measured.

The present measurement technique involves sampling the waveform atvarious points. It is assumed for illustrative purposes, that only thehigh or low values are measured when sampled and further that thetransition from high to low or low to high is always instantaneous. Nomeasurement is to be performed at some unknown middle transition when itswitches from high to low or vice versa. For the ideal waveform, thisimplies that as long as the same frequency is sampled as the waveform's,the same result will always be obtained.

FIG. 7 illustrates two examples of the ideal waveform being measured.

Hereinafter, and in all the figures, the dotted lines indicate thepoint(s) at which the waveform is sampled. The arrow(s) below and theletters “H” or “L” indicate if the result at that point is High (H) orLow (L). The solid vertical lines show the boundaries of each period. InFIG. 7A the waveform is sampled when the waveform is at low. In FIG. 7B,the waveform sampled is always high. Note that for FIG. 7A eachsubsequent period is sampled at the same relative location within theperiod. The same applies to FIG. 7B.

Now, comparing the aforementioned waveforms to one that is not ideal,when the jittering waveform is sampled at a given frequency, the samevalue will not always be measured. Depending on where the sample istaken, it may be at low or high, or only sometimes at low or high. FIG.10 compares the ideal waveform to the jittering waveform, with bothmeasured at the same points. In FIG. 10A, i.e., the ideal waveform, thevalues are all shown to be at low. However, in FIG. 10B, the measurementis not always at low even when sampled at the same position as in FIG.10A. Periods 2, 3, and 4 are at high when measured, as a result of thejitter.

Thus, jitter in the jittering waveform is detected when the measurementis taken.

At the measurement point illustrated in FIG. 10B, there is shown thatthree measurements out of five are at high. One may conclude that thereexists ⅗ or a 60% probability of finding the waveform at high. If themeasurement were taken earlier or later in each period, the percentageof highs and lows would change. That percentage of high and lowmeasurements is indicative of the number of times the edge of thewaveform was on one side or the other of the sample point. It is furtheralso related to the jitter and it is employed to measure it and evaluateit. Sampling at multiple different points yields a mathematicaldescription of the probability of the waveform being at high at anygiven point. Accordingly, a measurement at the same location in a periodtaken across multiple periods may be advantageously used to describe theprobability of the waveform being on one side or the other of the samplepoint. By taking many measurements at various points in each period, onemay determine the probability of the waveform being at high or low atany point in the period. FIGS. 8 through 11 illustrate this occurrence.In each of the figures, the value is measured across five periods. Ateach time, the location for measuring the value is moved slightly to theright in the period.

Referring to FIG. 8, the waveform is measured when it is always at low.The ideal waveform in FIG. 8A and the jittering waveform in FIG. 8Byield the same result. Periods 1 through 5 when measured are identical.This is not the case in FIG. 9, where the measurements differ. WhileFIG. 9A shows the ideal waveform always measuring at low, FIG. 9B showsthat period 3 measures at high. FIG. 10 shows similar results. In FIG.10A, the ideal waveform is always low even though it is substantiallynear to the transition point. In FIG. 10B, high values are measured inperiods 2, 3, and 4. Finally, in FIG. 11, the boundary of the jitter haspassed. Both FIGS. 11A and 11B, show that every measurement remains athigh.

For the measurements illustrated in FIGS. 8B and 11B in which thejittering waveform is always depicted at high or at low, there is shownthe minimum and maximum jitter. The middle two, FIGS. 9B and 10B, arewithin the boundaries of the jitter and the values measured are usedsubsequently for further analysis.

For the examples depicted in FIGS. 8 through 11, the data thatsummarizes the jittering waveform is shown in the Table 1 below: TABLE 1High/Total Probability H = 0 times L = 5 times 0/5 = 0% H = 1 time  L =4 times 1/5 = 20% H = 3 times L = 2 times 3/5 = 60% H = 5 times L = 0times 5/5 = 100%

TABLE 2 Probability of High Jitter 0% (0%-0%) 0% 20% (20%-0%)  20% 60%(60%-20%) 40% 100% (100%-60%)  40%

The number of high measurements divided by the number of samples givesthe probability of the measurement being at high. This probability isrelated to the jitter even though it is not the actual jitter. Thejitter is calculated by taking the difference in the edge locations ofthe ideal versus the jittering waveform. The distance that the edgemoves represents the actual jitter.

To further clarify, and with reference to FIG. 12, there is shown thetwo waveforms, respectively in FIGS. 12A and 12B, with the waveformssampled at 10 points. Between points 3 and 4 in FIG. 12A, a transitionfrom a low to a high takes place. In FIG. 12B, the transition from lowto high occurs between points 7 and 8. FIGS. 12A and 12B show the samewaveform showing transitions at point 4 in FIG. 12A, and at point 8 inFIG. 12B. The difference between the positions of the two pointsrepresents the jitter. By way of example, if each measurement point inFIG. 12 is 0.1 second apart, then point 4 in FIG. 12A and point 8 inFIG. 12B are 4 points or 4×0.1 sec apart. Accordingly, there are 0.4seconds of jitter in the waveform. Now, to further clarify itssignificance, and referring to point 9 in FIG. 12B, if the jitter ismeasured by way of point 8, the question arises as to whether point 9should also be used to obtain another jitter value of 0.5 seconds. Theanswer is negative, and as such, the first transition of low to highbecomes significant, while the following highs are not because they donot describe the jitter. Accordingly, it may be stated that at point 9in FIG. 12B there is a 100% chance of the waveform being at high, whichis illustrated in Table 1, although it cannot be consider jitter.Consequently, it becomes necessary to add a column to Table 1 to takethis into account. If one were to subtract high values which do not showthe jitter then the data would be deemed to be useful. Thus, any otherhigh value in FIG. 12B that is measured, apart from point 8, can bediscarded, retaining only the useful measurement attributed to point 8.

Referring again to the previous example with reference to FIGS. 8through 11 and Table 1, and having now a clearer understanding of whatthe numbers represent, the chart is revised to show the actual jitter.With reference to Table 2, one may observe the number of times thewaveform is at high. The second column represents the new value showingthe actual jitter. It is obtained by taking column 1 and subtractingfrom each value the preceding value. For example, considering theprobability of a high from rows 2 and 3 in Table 2, the resulting valuesare 20% and 60%, respectively. Subtracting 20 from 60 results in 40%,which is the value that is to be placed in the jitter column in thethird row. 40% in the jitter column indicates that 40% of the edges falluniquely into the measurement bracket. These are the edges or transitionpoints of the waveforms.

By measuring more periods at each measurement position and increasingthe number of measurement positions as well, it is possible to measurethe jitter very accurately. FIG. 13 shows a waveform for 1 millionperiods. This number of periods has been shown empirically and is deemedto be a suitable number for accurate measurement without incurring in anexcessive measurement time. In FIG. 14 the same 1 million periods areshown in a different format. All the waveforms are identical. Now,assuming that jitter is detected in the waveform in FIG. 13. The result,when displayed in the same format as in FIG. 14, is shown in FIG. 3,wherein one waveform is measured over 1 million periods. In each period,the edge of the wave is at a different location. Instead of themeasurement points used in the previous example additional measuringpoints a taken to obtain a finer resolution. The measurement points arelabeled across the bottom of FIG. 3. There are 250 measurements takenacross each period and labeled Measurement Points 1 through 250. Threeregions are also labeled. Region 1 shows the area where none of thewaveforms are at high. Measurement points 1 through 10 are in thisregion where they are always at low across the one million periods. InRegion 2, the waveforms are sometimes at high and sometimes at low. InRegion 3, the area where all of the waveforms measure at high has beenreached. The significant region is Region 2 which is where the jitterexists. The left and right boundaries of Region 2 are respectively theminimum and the maximum jitter locations. All of the low to hightransitions of the waveforms happen within Region 2.

Now, with reference to FIG. 15 depicting a chart plotting the number ofhighs measured versus measuring points taken at 10 picosecond intervalsfor 1 million periods sampled at each position, there is shown differentdata that does not exactly match FIG. 3. The X axis shows themeasurement position from 1 to 250 with each position taken at 10picoseconds intervals. The Y axis indicates the number of times thevalue measures a high at that position. With particular reference tomeasurement point 1 on the far left of the X axis in FIG. 15 and withreference to FIG. 3 one observes where Measurement Point 1 is locatedwithin the period. One million periods are sampled at that sameposition. At each sample point it is determined whether the waveform isat low or at high. If the waveform is at high, it is added to the totalnumber of highs measured. After 1 million periods, at Measurement Point1 no high value has yet been measured. Thus, the total is 0. The Y axisin FIG. 15 shows that at point 1, there are 0 highs being measured. Asone proceeds through measurement points 2, 3, 4, still no high valuesare detected across 1 million cycles. These measurement points form partof Region 1. Finally, at approximately Measurement Point 100, one beginsto find a high value. This becomes the left boundary of what is labeledRegion 2. As one continues to move to the right, more and more periodsare found to be at high at each particular measurement point. Observingmeasurement point 125 on the X axis, it is determined that forMeasurement Point 125 on the ascending line, “Highs Measured”corresponds roughly to 500,000 on the Y axis, “Number of HighsMeasured.” This indicates that at Measurement Point 125 a high value ismeasured approximately 50% of the time, or 500,000 out of 1,000,000samples. As one proceeds across the X axis towards 156, the number ofmeasured highs approaches 1,000,000. This is the point wheremeasurements are taken over 1,000,000 periods and finding every one tobe at high. This is the boundary of Region 3. One will never measuremore than 1,000,000 since it is a sampling of 1,000,000 periods, and sothe graph is limited at the top. Once a stable 1 million reading hasbeen taken the far side of the jitter has been reached.

As one proceeds with the sampling, it is intuitive that as one movesfrom Region 1 where no waveform at high is found towards Region 3 wherethe waveform is always at high, an increasing number of high values willbe encountered. This increase is illustrated in FIG. 15. In addition,the “Unique Edges” shaped as a Gaussian curve are shown concurrently inthe Figure. This corresponds to the “Jitter” column in Table 2 which wasdiscussed previously.

Referring now to previous discussion of Table 2 for the explanation ofthese numbers, it is observed that in FIG. 15 the “Unique Edges” curveat Measurement Point 125. At this point the number of “Highs Measured”is roughly located at the 100,000 mark on the Y axis. What thisindicates is that at Measurement Point 125 there are 100,000 out of1,000,000, or 10% of the edges that uniquely jittered at that point. Ifone measures at Measurement Point 125 over 1,000,000 periods one willfind the waveform high 50% of the time (from “Highs Measured”) but only10% of the time will they be uniquely transitioning from low to high atMeasurement Point 125.

At this point it is now possible to begin a statistical analysis. Tocalculate the mean the measurement point is first to be adjusted.Although points 1 through 250 have been labeled, in actuality they aretime based measurements. Each measurement is 10 ps apart, and for thenext steps they will be used in the same format. The Measurement Pointsin this table below are the time values. By way of example, MP₁ in thetable below is 1×10 ps, or 10 ps. MP₃ is 3×10 ps, or 30 ps. When,finally, the results are obtained for the mean and standard deviationthey will be in picoseconds. Measurement Point Number of Unique Highs(Jitter) MP₁ J₁ MP₂ J₂ MP₃ J₃ MP_(X) J_(X)The standard formula for calculating the mean is:${{Mean} = \frac{\sum X}{N}},$

where X are the individual values and N is the sample size, or number ofvalues.

The above formula must be adjusted slightly. For each Measurement Point,one million samples are preferably taken, thus weighting eachMeasurement Point. The value X in the standard formula is replaced by(MP_(X) times J_(X)). Similarly, the number of samples in thedenominator needs to be adjusted. It is not 250, the number ofmeasurement points, but instead the total number of highs measured. Thenew formula is shown below.${Mean} = \frac{\sum\limits_{1}^{X}\left( {{MP}_{X}*J_{X}} \right)}{\sum\limits_{1}^{X}J_{X}}$

Recalling that at the beginning of the present discussion, T₀ wasdefined to be the mean value of the plurality of measurements, for thediscussion that follows hereinafter, calculation of the mean will beomitted, and an estimate of its location places it at Measurement Point125. Further analyzing FIG. 15, recalling that each measurement pointstands 10 picoseconds apart, at Measurement Point 125 one is positionedat 125×10, or 1250 picoseconds into the period. One can then approximatethe extent of the jitter. Region 2 in FIG. 15 begins at measurementpoint 100, ending at measurement point 156 (both approximations). So,the minimum jitter in the period is at point 100, i.e., 1000 picosecondsinto the period, and the maximum jitter is 1560 picoseconds into theperiod. The total jitter is 1000-1560, or 560 picoseconds wide. (Notethat minimum and maximum refer to the first and last locations within aperiod wherein a transition is found). The jitter at any particularmeasurement point may be determined by calculating the difference of itslocation in time from that of T₀, as previously discussed.

From the data collected it is further determined that: Minimum jitter1000 ps Maximum jitter 1560 ps Total jitter  560 ps

The next piece of information that can be calculated is the standarddeviation.

Using this mean one now calculates the standard deviation. The standardformula for calculating the standard deviation is as follows. Using theunbiased equation with N−1 instead of N in the denominator${{Standard}\quad{Deviation}} = \sqrt{\frac{\sum\left( {X - {mean}} \right)^{2}}{N - 1}}$In the case once again X is replaced by (MP_(X) times J_(X)) and N withthe sum of J values. The resulting equation is shown below.${{Standard}\quad{Deviation}} = \sqrt{\frac{\sum\limits_{1}^{X}\left( {\left( {{MP}_{X}*J_{X}} \right) - {mean}} \right)^{2}}{\left( {\sum\limits_{1}^{X}J_{X}} \right) - 1}}$

The numbers calculated must then be correlated to the hardware beingtested. After testing multiple chips a correlation between good or badchips and a certain level of jitter is found and the tester isprogrammed to sort out chips as good or bad depending on the level ofjitter. These criteria are determined from real data and vary for eachapplication of the present invention. It has been found that thestandard deviation and the total (or peak) jitter are most useful inevaluating the chips tested.

For illustrative purposes, the example above was implemented using aTeradyne J973 tester. This method applies to other test platforms aswell. On the J973 the implementation was as follows:

The tester is known to include a test pattern generator which containsthe desired states of the various pins for the duration of the test, anda timing generator which takes the states of the pattern generator andplaces them precisely within the period, either for inputting data tothe chip, or for reading values from the chip. The tester firstinitializes the chip and configures the PLL to run at the desiredfrequency. Then, the measurement circuitry reads the values in eachperiod at a predetermined location (FIG. 2, Step 101). The tester cyclesthe chip continuously for a number of cycles to read one set ofmeasurements (Steps 102 through 105). In the present example, 1 millioncycles were chosen. The tester measures the pass and fails based on acomparison between the expected output of the chip and an actual outputof the chip. In this instance, the tester was programmed to a lowoutput. Thus, when the output of the PLL measured a high at the measuredpoint it was labeled as a “fail.” This is not indicative of an actualfail, simply a necessary way of programming due to the limitations ofthe test system and its intended use. These “fails” are in essence thehigh values collected throughout the test. Generally, the tester isprogrammed to detect a high, or fail, after the level of the PLLwaveform passes a given threshold, e.g., 0.6 volts, or one-half the 1.2volt peak-to-peak signal.

When the first 1 million measurements were collected, the testeranalyzes the collected data to determine how many highs there are andthat data is adjusted to find the unique edges (Step 106). Thismeasurement is saved in the tester memory in an array and the testproceeds to the next measurement point. At the second measurement point,it is not necessary to reinitialize the chip, simply to continue themeasurements as before, reading 1 million values and storing them in thememory. This process continues for as many measurement points asnecessary until END is reached.

It is at this point that the tester determines the statistical values,the mean, minimum, maximum, standard deviation, and the like. These arecalculated by the tester code, although other calculations can beprogrammed as well. The resulting values may also be stored in anoff-tester database for analysis for future comparison to other chips.The values are then taken by the tester and compared to predeterminedlimits to determine if the chip is good or bad. In some cases, thestandard deviation is used as a sorting criterion, in others the minimumor maximum, or the total jitter. The PLL may also be run at multiplefrequencies with different configurations and those results compared.Regardless of the number of criteria, the tester compares its calculatedvalues and determines sorting the chips accordingly. It is also possibleto sort chips according to their jitter, rather than simplydistinguishing as good or bad. Chips of like jitter may be grouped intodifferent categories, allowing the criteria used to vary widely based onthe specific application to which the method was applied.

The method as described presents numerous advantages, which include:

-   -   1) Measuring the jitter on repetitive signals such as clock        signals by using existing testers;    -   2) Eliminating the need for external instrumentation and        associated cost,    -   3) Quantitatively analyzing the jitter; and    -   4) Easily sorting chips as good or bad based on the results of        the jitter measurement.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, the embodiments of the invention as set forth aboveare intended to be illustrative, not limiting. Various changes may bemade without departing from the spirit and scope of the invention asdefined in the following claims.

1. A method for testing a chip by measuring jitter on repetitivewaveforms generated by a circuit integral to the chip under test, thewaveforms being inputted into a tester coupled to the chip under test,the method comprising the steps of: a) determining where the waveformgenerated by the circuit undergoes a transition; b) taking a pluralityof measurements from the waveform at periodic time intervals, saidmeasurements being taken in a region where the waveform undergoes saidtransition; c) analyzing the measurements taken from said region toquantify said transition and calculating a jitter on said waveform; andd) having said tester sort the chip under test according to said jitterand record results of said sorting in a recording device.
 2. The methodas recited in claim 1, wherein step b) further comprises taking saidplurality of measurements in regions where i) the waveform is always atlow, ii) the waveform is always at high and iii) the waveform undergoessaid transition.
 3. The method as recited in claim 1 wherein saidanalysis comprises calculating the mean and standard deviation of saidmeasurements.
 4. The method as recited in claim 1 wherein said analysiscomprises determining a minimum and a maximum value where said waveformundergoes said transition.
 5. The method as recited in claim 1 whereinsaid analysis further comprises the step of accumulating saidmeasurements of the waveform taken in the region where said transitionsoccur.
 6. The method as recited in claim 1 wherein said measurementstaken at periodic time intervals are ordered in a queue.
 7. The methodas recited in claim 6, wherein said measurements are correlated to thelocation within the period of the waveform where each measurement wastaken.
 8. The method as recited in claim 6 wherein said correlation isobtained by multiplying the position in said queue of each of saidmeasurements by the length of said time interval.
 9. The method asrecited in claim 6 wherein jitter is measured as a distance by which anedge location of a jittering waveform is displaced with respect to theedge location of an ideal waveform, said jitter being the difference inthe edge location of said ideal waveform from the edge location of saidjittering waveform.
 10. The method as recited in claim 9, wherein saidideal waveform is a waveform having a transition which is positioned ata statistical mean of a plurality of synchronized waveforms having thesame period.
 11. The method as recited in claim 10, wherein said idealwaveform is stored in said tester.
 12. The method as recited in claim 1.wherein in step d) said sorting comprises the steps of: a) gathering andanalyzing statistical data obtained from good and bad chips previouslytested; and b) correlating the statistical data against the data of achip under test.
 13. The method as recited in claim 1 wherein in stepa), said transition is detected when a predetermined voltage thresholdis crossed.
 14. A method for testing a chip by measuring jitter onrepetitive waveforms generated by a circuit integral to the chip undertest, the waveforms being inputted into a tester coupled to the chip,the method comprising the steps of: a) determining where the waveformgenerated by the circuit undergoes a transition, said transition takingplace when said waveform crosses a predetermined voltage threshold; b)taking a plurality of measurements from the waveform at periodic timeintervals, said measurements being taken in regions where i) thewaveform is always at low, ii) the waveform is always at high and iii)the waveform undergoes said transition; c) analyzing the measurementstaken from said regions and calculating a jitter on said waveform, saidjitter being determined by a distance by which an edge location of ajittering waveform is displaced with respect to the edge location of anideal waveform, said jitter being the difference in the edge location ofsaid ideal waveform from the edge location of said jittering waveform,said ideal waveform being a waveform having a transition which ispositioned at a statistical mean of a plurality of synchronizedwaveforms having the same period; d) having the tester sort the chipsaccording to said jitter by gathering and analyzing statistical dataobtained from good and bad chips previously tested, correlating thestatistical data against the data of a chip under test; and recordingresults of said sorting in a recording device.
 15. A program storagedevice readable by a machine, tangibly embodying a program ofinstructions executable by as machine to perform method steps fortesting chips by measuring jitter on repetitive waveforms generated by acircuit integral to the chip, the waveforms being inputted into a testercoupled to the chip, said method steps comprising: a) determining wherethe waveform generated by the circuit undergoes a transition; b) takinga plurality of measurements from the waveform at periodic timeintervals, said measurements being taken in a region where the waveformundergoes said transition; c) analyzing the measurements taken from saidregion to quantify said transition and calculating a jitter on saidwaveform; and d) having said tester sort said chips according to saidjitter and record results of said sorting in a recording device.
 16. Asystem for testing a chip by measuring jitter on repetitive waveformsgenerated by a circuit integral to the chip under test, the systemcomprising: a) a tester coupled to said chip under test; and b) amicroprocessor comprising: i) means for determining where the repetitivewaveform generated by the circuit undergoes a transition; ii) means fortaking a plurality of measurements from the repetitive waveform atperiodic time intervals, said measurements being taken in a region wherethe waveform undergoes said transition; iii) means for analyzing themeasurements taken from said region to quantify said transition andcalculating a jitter on said waveform, said tester sorting said chipsunder test according to said jitter.
 17. The system as recited in claim16, wherein in step c) said plurality of measurements are taken inregions where i) the waveform is always at low, ii) the waveform isalways at high and iii) the waveform undergoes said transition.
 18. Thesystem as recited in claim 16, wherein said means for analyzing saidmeasurements further comprises means for calculating the mean andstandard deviation of said measurements.
 19. The system as recited inclaim 16, wherein said means for analyzing said measurements furthercomprises means for determining a minimum and a maximum value where saidwaveform undergoes said transition.
 20. The system as recited in claim16, wherein said means for analyzing said measurements further comprisesmeans for accumulating said measurements of the waveform taken in theregion where said transitions occur.